The invention concerns a linear guide with an air bearing.
For the assembly of semiconductor chips automatic assembly machines known as xe2x80x9cDie Bondersxe2x80x9d are used which serve to mount the numerous, uniform chips of a wafer, which are located next to each other on a carrier, one after the other onto a substrate, e.g., a metallic leadframe. Coordinated with the pick-and-place movement of a chip gripper, a wafer table on which the chip carrier is located presents a next chip and the substrate is likewise shifted in order to present a new substrate position at a second location. In order to pick and subsequently place the chip, the chip gripper can be raised and lowered in a known way. U.S. Pat. No. 5,163,222 discloses a die bonder and U.S. Pat. No. 6,185,815 discloses a pick and place apparatus of the die bonder. Both patents are incorporated by reference.
Extremely high demands are set on such Die Bonders, in particular on the bearing arrangement of the moved parts. For the further processing of the mounted chips, they must be placed with positional accuracy on the substrate which requires the correspondingly accurate arrival of the chip gripper at the second location and already presupposes the accurate approach to the first location for picking the chip. On the other hand, high speeds and short cycle times are required which cause correspondingly high accelerations and forces of inertia to occur on the moved parts.
Up to now, various lever mechanisms which partially contain rocker guides have been used to produce the alternating movements of the chip gripper. Because of the considerable forces occurring on them, guides of this type are disadvantageous for a precise movement sequence and must be appropriately maintained. With another known mechanism, the chip gripper sits on the end of a lever which moves back and forth, i.e., corresponding to the slewing deflections of the lever, it experiences an arc-shaped movement which must always be stopped at the limit positions whereby a strong tendency to oscillations exists. A disadvantage of such lever-operated mechanisms lies in that they only allow transport of the chip in a fixed, predetermined path from a location A to a location B.
Driving mechanisms are also known with which the chip gripper is driven by means of a toothed belt. The disadvantage here is the large inaccuracy of placing the chip on the substrate.
From Japanese Patent application JP 01 194847 an air bearing is known with which a refrigerator is used to cool the air bearing. Such a cooling system has several drawbacks in that it makes the air bearing heavy and complicated.
The object of the invention is to develop a drive for the Pick and Place system of a Die Bonder with which the transport of a semiconductor chip allows high placement accuracy over any distances.
The named object is solved in accordance with the invention by means of a linear guide with an air bearing for the Pick and Place system of a Die Bonder.
A linear guide comprises a support element and a carriage sliding on the support element. The carriage has two hollow cylinders sliding on an air bearing on the support element and a body arranged between them. The support element and the two hollow cylinders of the carriage are separated by a gap to which compressed air can be applied. A linear motor formed from a u-shaped stator and an armature is foreseen as drive mechanism for the carriage. During operation, heat dissipation occurs in the armature which heats the carriage. In order to maintain changes in the thickness of the gap within predetermined limits, the support element and/or those parts of the carriage whose temperature influence the thickness of the gap are heated to a predetermined temperature.
In the following, embodiments of the invention are explained in more detail based on the drawing.